In electrical apparatus shielded by metal cladding, such as a circuit breaker for example, the molecular sieve used to absorb moisture and decomposition products formed when interrupting, is an aluminosilicate in the form of small crystals or small sticks. The water-hungry molecular sieve is placed in a tubular metal box which is flange-mounted on the neck of an orifice provided through the metal cladding of the apparatus. Prior to commissioning such apparatus, during shipping and storage, a closure plate is placed between the tube containing the molecular sieve and the cladding to avoid contact between the molecular sieve placed in the tube and the air contained inside the cladding. When the electrical apparatus is commissioned, the closure plate is removed to put the inside volume of the tube containing the molecular sieve into communication with the inside volume to be treated of the cladding, and then the apparatus is evacuated prior to being filled with dielectric gas. Removing the closure plate during commissioning of the apparatus thus requires the flange-mounting to be taken apart and then reassembled to hold the tube to the cladding.